This is one of my favourite Edgar Allan Poe stories. The cipher is just a simple monoalphabetic substitution one based on frequency analysis.

The same idea was "reused" by Arthur Conan Doyle in his story "The Adventure of the Dancing Men" where he used pictures of dancing men for his cipher text.

Note that Conan Doyle indirectly acknowledged Poe's influence (via his character Auguste Dupin) on Sherlock Holmes in "A Study in Scarlet".

Poe wrote three stories starring his detective Auguste Dupin all of which are fantastic. They are "The Murders in the Rue Morgue", "The Mystery of Marie Roget" and "The Purloined Letter". They are a must-read for all fans of detective fiction. Folks who have read Sherlock Holmes can see how much he was influenced by Dupin. The stories often have some close/clever observations/reasonings which given Poe's style of writing are a delight to read.

For example the opening paragraph of "The Murders in the Rue Morgue" reads;

THE mental features discoursed of as the analytical, are, in themselves, but little susceptible of analysis. We appreciate them only in their effects. We know of them, among other things, that they are always to their possessor, when inordinately possessed, a source of the liveliest enjoyment. As the strong man exults in his physical ability, delighting in such exercises as call his muscles into action, so glories the analyst in that moral activity which disentangles. He derives pleasure from even the most trivial occupations bringing his talent into play. He is fond of enigmas, of conundrums, of hieroglyphics; exhibiting in his solutions of each a degree of acumen which appears to the ordinary apprehension præternatural. His results, brought about by the very soul and essence of method, have, in truth, the whole air of intuition.

The essence of why some people love solving problems!